1. Field of the Invention
The present invention relates to a composite device system in which a first device and a second device are connected.
2. Description of the Related Art
In recent years, many portable apparatuses such as digital cameras have a lithium-ion battery. In general, it is difficult to detect a battery remaining amount using the battery voltage. Therefore, the battery remaining amount is measured by accumulating charge-discharge currents of the battery (refer to Japanese Laid-Open Patent Application No. 2001-174534). Also, a typical lithium-ion battery is vulnerable to over-charging and over-discharging. As such, it is necessary to protect such a lithium-ion battery from over-charging and over-discharging. Thus, the lithium-ion battery is used as a form of a battery pack including a battery monitor IC (Integrated Circuit) for monitoring the battery remaining amount and a protection IC for protecting from over-charging and over-discharging.
In a conventional battery pack including a battery monitor IC and a protection IC, the battery monitor IC includes a booster circuit for boosting a supplied voltage into a higher voltage. But, there is a problem in that a chip area of such a booster circuit becomes large in the battery monitor IC.